The present invention relates to control apparatus for high voltage D.C. transmission systems, for permitting connection of several three-phase current mains having different frequencies, of the type including at least two units operating in parallel during normal operation and each composed of a rectifier group and an inverter group, arranged so that, in unusual situations, one of the units is operated with its energy direction reversed. In such systems, especially for across-line couplings consisting of several units, it is desirable that when the total power is not utilized one of the units be operated to conduct energy in the reverse direction in order to produce a greater than normal total reactive power usage.